This invention relates to clutches used to remotely control the operation of hydraulic or pneumatic valves, such as the control valves of a crane or other apparatus. More particularly, the invention relates to a device for centering a clutch arm of such a clutch to reduce the effect of mechanical wear on its operation.
Many heavy lifting machines, such as cranes, and heavy earth moving machines, such as backhoes and excavators, use hydraulic pistons to power the movement of their various parts. Each of these hydraulic pistons is usually controlled by a hydraulic valve, which may be operated by a valve arm that can stroke the valve in two opposing directions to either extend or contract the associated hydraulic piston.
The valve arm may be operated by using a direct manual control or by a remote control. Manual operation is done by directly manipulating the valve arm (i.e. by raising and lowering or by moving it from side to side) by means of a handle. Remote operation is often accomplished by using a mechanical, electromechanical or electromagnetic clutch to couple a drive motor usually through a gear box to the valve arm. A remote control device may be used to manipulate the motor, thereby operating the valve arm.
Over time, linkages coupling the clutch to the valve arm may wear, causing a mechanical deadband to develop. The clutch arm will tend to sag downward (or to one side) due to gravity or another force. When the remote control device is used to manipulate the valve arm by raising it, the mechanical deadband reduces the effective distance by 30 which the valve arm is raised, producing an understroked response. When the remote control device is used to manipulate the valve arm by lowering it, the motor will overstroke the valve arm and try to force it past its maximum position. As a result, the valve arm may be damaged. Alternatively, the valve arm may be stopped by a dead stop and the clutch, the motor or the gear box may be damaged.
To deal with the problems caused by this sagging and the resulting mechanical deadband, attempts have been made to increase the range by which the remote control may manipulate the valve arm. This permits the motor to raise the valve arm a greater distance and thereby addresses the problem of understroking in one direction. However, this solution has no effect on the problem of overstroking in the opposite direction. Furthermore, this solution may require repetitive adjustment of the range by which the remote control may manipulate the valve arm, to avoid overstroking in both directions when the clutch and the associated linkages are new, and then to avoid understroking as the linkages wear.
Accordingly, there is a need for a device to reduce the effect of the mechanical deadband which results from wear on the linkage parts between the clutch and the valve arm.
The present invention provides a clutch arm centering assembly which biases a clutch arm of a clutch to a fixed neutral position. The clutch arm is coupled to a valve arm which controls a piston. As parts of the clutch arm wear, the clutch arm centering assembly ensures that the clutch arm does not sag, thereby avoiding problems relating to understroking and overstroking.
In one embodiment of a clutch arm centering assembly according to the present invention, a coil spring provides a lifting force to the clutch arm to prevent it from sagging as the coupling wears.
In a second embodiment of a clutch arm centering assembly according to the present invention, a helical spring is mounted to a hub attached to the clutch. The helical spring has a pair of spring arms which engage a centering pin fastened to the clutch arm from opposing sides to bias the clutch arm into a central position.
According to one aspect of the present invention, there is provided a clutch having a clutch arm coupled through a linkage to a valve arm of a valve, said valve arm having a neutral position in which said valve is closed and a first operative position in which said valve is open, said clutch arm having a first position corresponding to said neutral position and having a second position corresponding to said first operative position, said linkage tending to wear in use such that said clutch arm sags away from one of said first and second positions, and a positioning spring coupled to said clutch arm for biasing said clutch arm towards said one position.
According to another aspect of the present invention, there is provided a clutch having a clutch arm coupled to a valve arm of a valve, said valve arm having a neutral position in which said valve is closed and two operative positions one on each side of said neutral position, said clutch arm having a first position corresponding to said neutral position and having second and third positions on opposite sides of said neutral position corresponding to said two operative positions respectively of said valve, and a positioning spring coupled to said clutch arm for biasing said clutch arm towards said first position.